Lubricating composition for hot-rolling steel

ABSTRACT

Disclosed is a lubricating composition for hot-rolling steel, which comprises a base oil or base grease and a heat-insulating agent. Especially, (A) an inorganic compound which is melted by an absorption of heat at a temperature lower than 1200° C. and (B) an inorganic powder which is not melted or decomposed at a temperature lower than 1200° C. and has a heat conductivity lower than 0.01 cal/cm.s. °C. at room temperature and a friction coefficient smaller than 0.7 are incorporated in specific amounts singly or in combination in the composition. This lubricating composition has an excellent heat-insulating effect and effectively prevents the thermal crown of a work roll.

This application is a continuation of now abandoned application, Ser.No. 07/915,403, filed Jul. 20, 1992, which is a continuation of nowabandoned application, Ser. No. 07/465,092, filed Feb. 13, 1990, whichis the national phase of PCT/JP89/00591, filed Jun. 13, 1989.

TECHNICAL FIELD

The present invention relates to a lubricating composition forhot-rolling steel. More particularly, the present invention relates to alubricating composition by which a transfer of heat to a work roll froma material to be rolled is prevented and the effect of reducing thethermal crown of the work roll is attained by incorporating a specificheat-insulating agent into a base oil or base grease.

BACKGROUND ART

In the conventional hot-rolling method, only roll-cooling water is usedfor protecting a roll, but now a rolling oil is used for reducing therolling load and decreasing wear of the roll, and an excellent effect isattained thereby.

The main object of the conventional lubricant for hot rolling is toreduce wear of a work roll and improve the roll surface, because therequirement for the quality of a rolled product is relatively moderateand the thermal crown of the work roll is not regarded as important.Nevertheless, recently, an increased quality of the product has beendemanded, and the effect of reducing the thermal crown of the work roll,which has a direct adverse influence on the quality of the product, hasbecome important.

DISCLOSURE OF THE INVENTION

The present invention relates to a lubricant composition for hot-rollingsteel, which is characterized in that a heat-insulating agent isincorporated into a base oil or base grease. More specifically, it wasfound that if two specific kinds of heat-insulating agents, i.e., (A) aninorganic compound which is melted by an absorption of heat at atemperature lower than 1200° C., and (B) an inorganic powder which isnot melted or decomposed at a temperature lower than 1200° C. and has aheat conductivity lower than 0.01 cal/cm.s. °C. at room temperature anda friction coefficient smaller than 0.7, are incorporated singly or incombination in specific amounts in the composition, an excellentheat-insulating effect can be attained and the thermal crown of a workroll can be effectively prevented. The present invention was completedbased on this finding.

In accordance with the first aspect of the present invention, there isprovided a lubricating composition for hot-rolling steel, whichcomprises a base oil and a heat-insulating agent incorporated therein,wherein the above-mentioned components (A) and (B) are used as theheat-insulating agent. By incorporating the heat-insulating agents (A)and (B) having different properties in combination in the base oil, theheat-insulating effect can be increased by the synergistic effect ofthese heat-insulating agents.

In accordance with the second aspect of the present invention, there isprovided a lubricating composition for hot-rolling steel, whichcomprises a base grease and a heat-insulating agent incorporatedtherein, and the above-mentioned component (A) and/or theabove-mentioned component (B) is used as the heat-insulating agent. Inthis aspect, the base grease is used instead of the base oil, and theheat-insulating property is improved by the combination of the basegrease with the component (A), the component (B) or the components (A)and (B). Since a grease has a low flowability at a high temperature,compared with an oil, a remarkable effect can be attained in the greaseby the addition of the component (A) or (B) alone. If the components (A)and (B) are incorporated in combination, a highest effect can beattained due to the synergistic action of the two components.

In the above-mentioned lubricating composition for hot-rolling, byincorporating an extreme pressure additive and/or a solid lubricanttogether with the heat-insulating agents (A) and (B), the lubricatingproperty of the base oil can be further improved, and the lubricatingproperty, heat-insulating property, storage stability, working property,and water washing resistance of the base grease can be further improved.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram illustrating the method of measuring the contactheat transfer ratio between metals.

BEST MODE OF CARRYING OUT THE INVENTION

The surface temperature of a work roll is elevated to about 800° C. bycontact with a material to be rolled. Most of the conventionallubricants for hot rolling comprise a mineral oil, an oiliness agent, anextreme pressure additive, and a solid lubricating agent, incombination, and although the lubricating property is taken intoconsideration, an insulation of heat (prevention of transfer of heat tothe work roll from the material to be rolled) is not considered.

Japanese Unexamined Patent Publication No. 60-6211 teaches that a rollcan be protected by adding a fine powder of an inorganic compound havinga melting point lower than 1200° C. under atmospheric pressure, anaverage particle size smaller than 1 μm, and no corrosive action on ironand steel including cast iron and cast steel and other metals, andacting as a substance having a poor heat conductivity to a commerciallyavailable hot-rolling oil (liquid).

The base oil disclosed in this patent publication is a commerciallyavailable hot-rolling oil (liquid) and is different from the base greaseused in the second embodiment of the present invention. Furthermore, thepowder used in the invention of the-above-mentioned patent publicationis a powder of an inorganic compound which melts at a temperature lowerthan 1200° C., and the heat transfer-preventing effect is drasticallyreduced after melting.

In the present invention, where a liquid base oil is used, an inorganiccompound melting at a temperature lower than 1200° C. and an inorganicpowder not melting or decomposing at a temperature lower than 1200° C.are used in combination, and where a base grease as semi-fluid greasehaving a higher heat-insulating property than that of the liquid baseoil is used, two kinds of the inorganic powders are added singly or incombination, whereby the heat-insulating property is further increased.

Namely, the heat-insulating agent used in the present invention includes(A) an inorganic compound which is melted by an absorption of heat at atemperature lower than 1200° C. and (B) an inorganic powder which is notmelted or decomposed at a temperature lower than 1200° C., is stableagainst oxidation and has a heat conductivity lower than 0.01 cal/cm.s.°C. at room temperature and a friction coefficient smaller than 0.7. Asthe inorganic compound (A) which is melted by an absorption of heat at atemperature lower than 1200° C., there can be mentioned solid powders ofcondensed phosphoric acid salts such as condensed salts of KPO₃ andNaPO₃ and K₄ P₂ O₇, sodium silicate, chromic acid salts such a K₂ Cr₂ O₇⁻, and halides such as NaCl, KCl, KF, KBr and KI. Condensed phosphoricacid salts and sodium silicate, which have no corrosive action on arolling mill or a material to be rolled, are especially preferable.

As the inorganic powder (B) which is not melted or decomposed at atemperature lower than 1200° C., is stable against oxidation and has aheat conductivity lower than 0.01 cal/cm.s. °C. at room temperature anda friction coefficient smaller than 0.7, there can be used boronnitride, silicon nitride, amorphous carbon, K₃ PO₄, Ca₃ (PO₄)₂,bentonite, SiO₂ and ZnO. The friction coefficient referred to herein isdetermined by the pin-on-disk method (a rod having a diameter of 3 mmand a flat top end is pressed under a load of 1 kgf against a diskhaving a diameter of 11 mm, and the disk is slid at a speed of 0.01 ms).

A heat-insulating agent having an average particle size smaller than 50μm can be used, but in view of the clearance between the roll and thematerial to be rolled, preferably the average particle size of theheat-insulating agent is smaller than 10 μm.

The reason why better results are obtained when the inorganic compound(A), which is melted by an absorption of heat at a temperature lowerthan 1200° C., and the inorganic powder which is not melted ordecomposed at a temperature lower than 1200° C. and has a heatconductivity lower than 0.01 cal/cm.s. °C. and a friction coefficientsmaller than 0.7 are used in combination, has not been completelyelucidated, but it is considered that the reason is probably as follows.At the rolling step, the temperature and pressure become high in a rollbite (higher than 600° C. and higher than 2000 kgf/cm²). At this point,the heat-insulating agent (A) is promptly melted by absorption of heatand prevents heat from being transferred to the work roll from thematerial to be rolled. It is known that the heat conductivity of aliquid is, in general, increased more than that of a powder.Accordingly, it is considered that the heat transfer-reducing effect ofthe heat-insulating agent (A) is abruptly decreased by melting. On theother hand, since the heat-insulating agent (B) is not melted ordecomposed even under high-temperature and high-pressure conditions, theheat-insulating agent (B) is present in the form of a powder in the rollbite and prevents the work roll from falling in contact with thematerial to be rolled, and it is considered that since the powder per sehas a lubricating property, a generation of heat by friction in the rollbite is reduced by the powder of the heat-insulating agent (B).

Namely, although the heat-insulating agent (A) has an excellentheat-insulating property, when the heat-insulating agent (A) is meltedat a high temperature, the heat-insulating property is drasticallyreduced. 0n the other hand, since the heat-insulating agent (B) is notmelted or decomposed at a temperature lower than 1200° C., theheat-insulating agent (B) has a heat-insulating property over a broadtemperature range. Accordingly, if the heat-insulating agent (A) and theheat-insulating agent (B) are made present at a specific ratio, alubricating agent having the excellent effects of both heat-insulatingagents (A) and (B) can be obtained.

Preferably, the heat-insulating agent is added in an amount of 5 to 50%by weight, especially 10 to 40% by weight. If the amount of theheat-insulating agent is smaller than 5% by weight, the heat-insulatingeffect is too low, and if the amount of the heat-insulating agent islarger than 50% by weight, the viscosity of the lubricant becomes toohigh and the oil-supplying property is degraded. Preferably, the ratioof the heat-insulating agent (A) to the heat-insulating agent (B) is inthe range of 49/1 to 1/49, especially 19/1 to 1/4. This is because, ifthe proportion of the heat-insulating agent (A) is reduced, theheat-insulating property is lowered by an absorption of heat in the rollbite, and if the proportion of the heat-insulating agent (B) is reduced,the heat-insulating property at high temperature is lowered.

As the base oil that can be used in the present invention, there can bementioned medium and heavy mineral oils such as spindle oil, machineoil, dynamo oil, motor oil, cylinder oil and bright stock, animal andvegetable oils such as beef tallow, lard, sperm oil, palm oil, coconutoil, linseed oil, rice bran oil and soybean oil, synthetic oils such asesters of fatty acids having 8 to 22 carbon atoms with monohydric orpolyhydric alcohols, α-olefins, polybutene, silicone oils and fluorineoils, and mixtures of these oils.

As the base grease that can be used in the present invention, there canbe mentioned lithium soap grease, calcium soap grease, sodium soapgrease, aluminum soap grease, calcium complex grease, polyurea greaseand organo-clay grease. Lithium soap grease, calcium complex grease,polyurea grease and organo-clay grease, which have an excellent heatresistance, are preferable.

As the solid lubricant that can be used in the present invention, therecan be mentioned inorganic solid lubricants such as graphite (naturalgraphite and artificial graphite), molybdenum disulfide, mica (naturalmica and artificial mica), fluorinated graphite, boron nitride, softmetals (such as gold, silver and copper) and talc, and organic solidlubricants such as PTFE (polytetrafluoroethylene), MCA (melaminecyanuricacid adduct) and phthalocyanine. Graphite (natural graphite andartificial graphite), mica (natural mica and artificial mica), boronnitride and talc, which have an excellent heat resistance and oxidationstability at a high temperature and have no substantial influence on amaterial to be rolled, are preferable. Preferably the amount added ofthe solid lubricant is 0 to 40% by weight, especially 5 to 15% byweight. If the amount of the solid lubricant exceeds 40% by weight, theviscosity of the lubricant becomes too high and the oil-supplyingproperty is reduced.

As the extreme pressure additive that can be used in the presentinvention there can be mentioned sulfur compounds, phosphorus compounds,chlorine compounds and organic metal compounds. Preferably the amountadded of the extreme pressure additive is 0 to 20% by weight, especially0.5 to 10% by weight. If the amount of the extreme pressure additiveexceeds 20% by weight, undesired side effects such as an appearance of acorrosive action and reduction of the stability of the micell structureof the grease occur.

The present invention will now be described in detail with reference tothe following examples and comparative examples.

EXAMPLES 1 THROUGH 26 AND COMPARATIVE EXAMPLES 1 THROUGH 5

A base oil, a base grease, a heat-insulating agent, an extreme pressureadditive, and a solid lubricant were mixed at a mixing ratio shown inTable 1, whereby lubricating agents of Examples 1 through 26 andComparative Examples 1 through 5 were prepared. With respect to each ofthe so-obtained compositions, the performances were evaluated accordingto the test methods described below. The results are shown in Table 1.

Lubricating Property Test by Hot Lubricating Property Tester Model E-12

According to the principle of the hot lubricating property tester ModelE-12, both ends of a test piece were fixed and the test pieceinduction-heated gripped between rolls while supplying an oil to thetest piece, and a slip lubrication effected. The friction coefficientand seizure resistance of each lubricant were examined to evaluate thelubricating property.

Friction coefficient μ=T/R.W

in which T represents a shaft torque, R represents a roll radius, and Wrepresents a load. The outlines of the tester and the test conditionsare as follows.

a) Type: lubricity tester of two-high type for slip lubricating

b) Roll dimension: 124 mm (diameter)×80 mm (length)

c) Roll material: high chromium roll (Hs=70-75)

d) Test piece material: SS-41 [20 mm (height)×20 mm (width)×580 mm(length)]

e) Test piece temperature: 400° C., 600° C. and 800° C. (automaticallyadjusted)

f) Revolution: 200 rpm

g) Rolling load: 500 to 3000 kgf (the load is increased by 500 kgf atevery time)

h) Method of supplying lubricant: applying

B) Rust Prevention Test

a) The test piece used at the test (A) was cut to a size of 20 mm×20mm×100 mm.

b) The test piece prepared at a) above was hung under the eaves andallowed to stand for 2 weeks, and the state of rusting was checked.

o: no rusting

x: extreme rusting

C) Water Washing Resistance Test

a) A defatted and weighed steel sheet (SPCC-SD 100 mm×100 mm×0.8 mm) wasuniformly coated with 30±3 mg of the lubricant.

b) The steel sheet prepared at a) above was washed with water under thefollowing conditions, and the weight was measured after the waterwashing and the residual oil ratio is determined.

a) Nozzle model number: 1/4 KBF 0865

b) Extrusion rate: 6.4 l/min (extrusion pressure=2.0 kgf/cm²)

c) Water washing time: 5 seconds (water temperature=25° C.)

d) Distance between steel sheet and nozzle: 200 mm

Residual oil or grease ratio (%)=[(amount of residual oil orgrease)/(amount coated of oil or grease)]×100

D) Measurement of Contact Heat Transfer Ratio between Metals

a) Material of test piece: wt-60 [25 25 mm (diameter)×50 mm (length)]

b) Temperature: 780° C. (high-temperature material), 22 to 30° C.(low-temperature material)

c) Thermocouple: CA (0.5 mm) sheath (attachment position=1.5 mm, 3.0 mm)

d) Heat-insulating material: kao wool

e) Compressive force: 500 kgf/cm²

f) Method of filling sample and thickness:

As shown in FIG. 1, a high-temperature material 3 was pressed against alow-temperature material 2 coated with a sample 1, and the contactinterface temperature of each sample and the heat flow flux werereckoned backward from the change of the temperatures of both materialswith a lapse of time after the contact. The cooling law of Newton wasapplied in an extended manner to determine the heat transfer coefficientbetween the metals. The obtained coefficient was compared with thecoefficient obtained when the sample is not coated, and the heattransfer ratio determined.

Note, in FIG. 1, reference numeral 4 represents a heat-insulatingmaterial and reference numeral 5 represents a thermocouple.

E) Roll Wearing Quantity Ratio in Actual Rolling Mill

Eight air spray nozzles (20 ml/min.nozzle) were attached to a work rollon the inlet side of F5 stand (6-stand mill), and about 300 tons of anordinary material rolled by using nickel grain rolls. The wear quantitywas measured and compared with the wear quantity observed when an oilingagent now available was used.

                                      TABLE 1-1                                   __________________________________________________________________________                                 Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                   1    2    3    4    5                            __________________________________________________________________________    Base Oil mineral oil (ISO VG 430)                                                                          85        85   85   85                                    synthetic oil (hydrocarbon type)                                                                       85                                          Base     lithium soap grease (mineral oil type)                               Grease   lithium soap grease (synthetic oil type)                                      polyurea grease (mineral oil type)                                            calcium complex grease (mineral oil type)                            Heat-    melting   (KPO.sub.3).sub.n                                                                       10   10   10   10                                Insulating                                                                             type      Na.sub.2 SiO.sub.4            10                           Agent    (A)       NaCl                                                                non-      silicon nitride                                                                         5    5              5                                     melting   amorphous carbon    5                                               type (B)  silicon dioxide          5                                 Extreme  zinc dialkyl dithiophosphate                                         Pressure tricresyl phosphate                                                  Additive sulfurized lard                                                      Solid    graphite (artificial)                                                Lubricant                                                                              mica (natural)                                                                boron nitride                                                                 MCA                                                                  Consistency Number (JIS K 2220)                                                                            --   --   --   --   --                           E-12 Model Hot                                                                         400° C.                                                                          load resistance (kgf)                                                                   2000 2000 2000 2000 2000                         Lubricating Per-   μ under above load                                                                   0.18 0.18 0.18 0.18 0.18                         formance Test                                                                          600° C.                                                                          load resistance (kgf)                                                                   1000 1000 1000 1000 1000                                            μ under above load                                                                   0.18 0.18 0.18 0.18 0.18                                  800° C.                                                                          load resistance (kgf)                                                                   500  500  500  500  500                                             μ under above load                                                                   0.18 0.18 0.18 0.18 0.18                         Rust Prevention Test         ∘                                                                      ∘                                                                      ∘                                                                      ∘                                                                      ∘                Water Washing Resistance Test (residual oil or                                                             45   46   45   44   46                           grease ratio; %)                                                              Contact Heat Transfer Ratio between Metals                                                                 0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                      Roll Wearing Ratio in Actual Rolling Mill                                                                  0.8-0.9                                                                            0.8-0.9                                                                            0.8-0.9                                                                            0.8-0.9                                                                            0.8-0.9                      __________________________________________________________________________

                                      TABLE 1-2                                   __________________________________________________________________________                                 Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                   6    7    8    9    10                           __________________________________________________________________________    Base Oil mineral oil (ISO VG 430)                                                                          85   82   80   77                                         synthetic oil (hydrocarbon type)                                     Base     lithium soap grease (mineral oil type)  90                           Grease   lithium soap grease (synthetic oil type)                                      polyurea grease (mineral oil type)                                            calcium complex grease (mineral oil type)                            Heat-    melting   (KPO.sub.3).sub.n                                                                            10   10   10   10                           Insulating                                                                             type      Na.sub.2 SiO.sub.4                                         Agent    (A)       NaCl      10                                                        non-      silicon nitride                                                                         5    5    5    5                                          melting   amorphous carbon                                                    type (B)  silicon dioxide                                            Extreme  zinc dialkyl dithiophosphate                                                                           3         3                                 Pressure tricresyl phosphate                                                  Additive sulfurized lard                                                      Solid    graphite (artificial)         5    5                                 Lubricant                                                                              mica (natural)                                                                boron nitride                                                                 MCA                                                                  Consistency Number (JIS K 2220)                                                                            --   --   --   --   1                            E-12 Model Hot                                                                         400° C.                                                                          load resistance (kgf)                                                                   2000 2500 2500 3000 2000                         Lubricating Per-   μ under above load                                                                   0.18 0.17 0.17 0.15 0.18                         formance Test                                                                          600° C.                                                                          load resistance (kgf)                                                                   1000 1500 2000 2000 1000                                            μ under above load                                                                   0.18 0.16 0.16 0.15 0.18                                  800° C.                                                                          load resistance (kgf)                                                                   500  1000 1000 1500 500                                             μ under above load                                                                   0.18 0.16 0.18 0.16 0.18                         Rust Prevention Test         x    ∘                                                                      ∘                                                                      ∘                                                                      ∘                Water Washing Resistance Test (residual oil or                                                             46   47   60   60   80                           grease ratio; %)                                                              Contact Heat Transfer Ratio between Metals                                                                 0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                      Roll Wearing Ratio in Actual Rolling Mill                                                                  0.8-0.9                                                                            0.5-0.6                                                                            0.6-0.7                                                                            0.5-0.6                                                                            0.8-0.9                      __________________________________________________________________________

                                      TABLE 1-3                                   __________________________________________________________________________                                 Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                   11   12   13   14   15                           __________________________________________________________________________    Base Oil mineral oil (ISO VG 430)                                                      synthetic oil (hydrocarbon type)                                     Base     lithium soap grease (mineral oil type)                                                            90   80   87   87   87                           Grease   lithium soap grease (synthetic oil type)                                      polyurea grease (mineral oil type)                                            calcium complex grease (mineral oil type)                            Heat-    melting   (KPO.sub.3).sub.n                                                                            10   10   10   10                           Insulating                                                                             type      Na.sub.2 SiO.sub.4                                         Agent    (A)       NaCl                                                                non-      silicon nitride                                                                         10   10                                                   melting   amorphous carbon                                                    type (B)  silicon dioxide                                            Extreme  zinc dialkyl dithiophosphate  3                                      Pressure tricresyl phosphate                3                                 Additive sulfurized lard                         3                            Solid    graphite (artificial)                                                Lubricant                                                                              mica (natural)                                                                boron nitride                                                                 MCA                                                                  Consistency Number (JIS K 2220)                                                                            1    1    1    1    1                            E-12 Model Hot                                                                         400° C.                                                                           load resistance (kgf)                                                                  2000 2500 3000 3000 3000                         Lubricating Per-   μ under above load                                                                   0.18 0.18 0.14 0.14 0.14                         formance Test                                                                          600° C.                                                                          load resistance (kgf)                                                                   1000 1500 2000 2000 2000                                            μ under above load                                                                   0.18 0.17 0.17 0.17 0.17                                  800° C.                                                                          load resistance (kgf)                                                                   500  1000 1500 1500 1500                                            μ under above load                                                                   0.18 0.17 0.16 0.16 0.16                         Rust Prevention Test         ∘                                                                      ∘                                                                      ∘                                                                      ∘                                                                      ∘                Water Washing Resistance Test (residual oil or                                                             80   85   81   81   81                           grease ratio; %)                                                              Contact Heat Transfer Ratio between Metals                                                                 0.7-0.8                                                                            0.6-0.7                                                                            0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                      Roll Wearing Ratio in Actual Rolling Mill                                                                  0.8-0.9                                                                            0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                                                                            0.7-0.8                      __________________________________________________________________________

                                      TABLE 1-4                                   __________________________________________________________________________                                 Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                   16   17   18   19   20                           __________________________________________________________________________    Base Oil mineral oil (ISO VG 430)                                                      synthetic oil (hydrocarbon type)                                     Base     lithium soap grease (mineral oil type)                                                            85   85   85   85   77                           Grease   lithium soap grease (synthetic oil type)                                      polyurea grease (mineral oil type)                                            calcium complex grease (mineral oil type)                            Heat-    melting   (KPO.sub.3).sub.n                                                                       10   10   10   10   10                           Insulating                                                                             type      Na.sub.2 SiO.sub.4                                         Agent    (A)       NaCl                                                                non-      silicon nitride               10                                    melting   amorphous carbon                                                    type (B)  silicon dioxide                                            Extreme  zinc dialkyl dithiophosphate                                         Pressure tricresyl phosphate                                                  Additive sulfurized lard                                                      Solid    graphite (artificial)                                                                             5                                                Lubricant                                                                              mica (natural)           5                                                    boron nitride                 5                                               MCA                                5                                 Consistency Number (JIS K 2220)                                                                            1    1    1    1    1                            E-12 Model Hot                                                                         400° C.                                                                          load resistance (kgf)                                                                   3000 3000 3000 2500 3000                         Lubricating Per-   μ under above load                                                                   0.15 0.14 0.14 0.18 0.15                         formance Test                                                                          600° C.                                                                          load resistance (kgf)                                                                   2000 2000 2000 1500 3000                                            μ under above load                                                                   0.18 0.18 0.18 0.18 0.15                                  800° C.                                                                          load resistance (kgf)                                                                   1500 1500 1500 1000 2000                                            μ under above load                                                                   0.17 0.17 0.17 0.17 0.17                         Rust Prevention Test         ∘                                                                      ∘                                                                      ∘                                                                      ∘                                                                      ∘                Water Washing Resistance Test (residual oil or                                                             86   86   87   86   86                           grease ratio; %)                                                              Contact Heat Transfer Ratio between Metals                                                                 0.6-0.7                                                                            0.6-0.7                                                                            0.6-0.7                                                                            0.7-0.8                                                                            0.6-0.7                      Roll Wearing Ratio in Actual Rolling Mill                                                                  0.6-0.7                                                                            0.6-0.7                                                                            0.6-0.7                                                                            0.7-0.8                                                                            0.5-0.6                      __________________________________________________________________________

                                      TABLE 1-5                                   __________________________________________________________________________                                 Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                   21   22   23   24   25                           __________________________________________________________________________    Base Oil mineral oil (ISO VG 430)                                                      synthetic oil (hydrocarbon type)                                     Base     lithium soap grease (mineral oil type)                                                            72   47                                          Grease   lithium soap grease (synthetic oil type)                                                                    72                                              polyurea grease (mineral oil type) 72                                         calcium complex grease (mineral oil type)                                                                             72                           Heat-    melting   (KPO.sub.3).sub.n                                                                       10   25   10   10   10                           Insulating                                                                             type      Na.sub.2 SiO.sub.4                                         Agent    (A)       NaCl                                                                non-      silicon nitride                                                                         10   10   5    5    5                                     melting   amorphous carbon                                                    type (B)  silicon dioxide                                            Extreme  zinc dialkyl dithiophosphate                                                                      3    3    3    3    3                            Pressure tricresyl phosphate                                                  Additive sulfurized lard                                                      Solid    graphite (artificial)                                                                             5    15   10   10   10                           Lubricant                                                                              mica (natural)                                                                boron nitride                                                                 MCA                                                                  Consistency Number (JIS K 2220)                                                                            1    2    1    1    1                            E-12 Model Hot                                                                         400° C.                                                                          load resistance (kgf)                                                                   3000 3000 3000 3000 3000                         Lubricating Per-   μ under above load                                                                   0.14 0.06 0.14 0.14 0.14                         formance Test                                                                          600° C.                                                                          load resistance (kgf)                                                                   3000 3000 3000 3000 3000                                            μ under above load                                                                   0.15 0.08 0.15 0.15 0.15                                  800° C.                                                                          load resistance (kgf)                                                                   2500 3000 2000 2000 2500                                            μ under above load                                                                   0.16 0.10 0.16 0.16 0.14                         Rust Prevention Test         ∘                                                                      ∘                                                                      ∘                                                                      ∘                                                                      ∘                Water Washing Resistance Test (residual oil or                                                             87   93   87   89   87                           grease ratio; %)                                                              Contact Heat Transfer Ratio between Metals                                                                 0.6-0.7                                                                            0.5-0.6                                                                            0.6-0.7                                                                            0.6-0.7                                                                            0.6-0.7                      Roll Wearing Ratio in Actual Rolling Mill                                                                  0.5-0.6                                                                            0.2-0.3                                                                            0.3-0.4                                                                            0.3-0.4                                                                            0.3-0.4                      __________________________________________________________________________

                                      TABLE 1-6                                   __________________________________________________________________________                                      Comparative                                                                          Comparative                                                                          Comparative                                                Example                                                                            Example                                                                              Example                                                                              Example                                                    26   1      2      3                             __________________________________________________________________________    Base Oil mineral oil (ISO VG 430) 90                                                   synthetic oil (hydrocarbon type)                                     Base     lithium soap grease (mineral oil type)                                                                        87     72                            Grease   lithium soap grease (synthetic oil type)                                      polyurea grease (mineral oil type)                                                                90                                                        calcium complex grease (mineral oil type)                            Heat-    melting   (KPO.sub.3).sub.n                                                                       10   10            10                            Insulating                                                                             type      Na.sub.2 SiO.sub.4                                         Agent    (A)       NaCl                                                                non-      silicon nitride                                                     melting   amorphous carbon                                                    type (B)  silicon dioxide              nickel                                                                        powder 10                                                                     (note 1)                      Extreme  zinc dialkyl dithiophosphate    3      3                             Pressure tricresyl phosphate                                                  Additive sulfurized lard                                                      Solid    graphite (artificial)           10     5                             Lubricant                                                                              mica (natural)                                                                boron nitride                                                                 MCA                                                                  Consistency Number (JIS K 2220)   --     --     --                            E-12 Model Hot                                                                         400° C.                                                                          load resistance (kgf)                                                                   2500 1500   2500   3000                          Lubricating Per-   μ under above load                                                                   0.18    0.18                                                                              0.16   0.14                          formance Test                                                                          600° C.                                                                          load resistance (kgf)                                                                   1500 500    1500   3000                                             μ under above load                                                                   0.17    0.17                                                                              0.16   0.16                                   800° C.                                                                          load resistance (kgf)                                                                   1000  500>  1000   2500                                             μ under above load                                                                   0.18 --     0.17   0.17                          Rust Prevention Test         ∘                                                                      ∘                                                                        ∘                                                                        ∘                 Water Washing Resistance Test (residual oil or                                                             80   40     80     87                            grease ratio; %)                                                              Contact Heat Transfer Ratio between Metals                                                                 0.7-0.8                                                                            0.9-1.0                                                                              0.9-1.0                                                                              0.8-0.9                       Roll Wearing Ratio in Actual Rolling Mill                                                                  0.8-0.9                                                                            0.9-1.0                                                                              0.7-0.8                                                                              0.7-0.8                       __________________________________________________________________________     Note 1                                                                        heat conductivity = 0.22 cal/cm · s · °C.            melting point = 1455° C.                                          

                                      TABLE 1-7                                   __________________________________________________________________________                                 Comparative                                                                          Comparative                                                                          Comparative                                                     Example 4                                                                            Example 5                                                                            Example 6                          __________________________________________________________________________    Base Oil mineral oil (ISO VG 430)                                                                          Commercial                                                                           Commercial                                                                           lubricating                                 synthetic oil (hydrocarbon type)                                                                  hot rolling                                                                          hot rolling                                                                          agent                              Base     lithium soap grease (mineral oil type)                                                            oil (A)                                                                              oil (B)                                                                              not added                          Grease   lithium soap grease (synthetic oil type)                                                          (oil used                                                                            (containing                                        polyurea grease (mineral oil type)                                                                at present)                                                                          10% of                                             calcium complex grease (mineral oil type)                                                                graphite)                                 Heat-    melting   (KPO.sub.3).sub.n                                          Insulating                                                                             type      Na.sub.2 SiO.sub.4                                         Agent    (A)       NaCl                                                                non-      silicon nitride                                                     melting   amorphous carbon                                                    type (B)  silicon dioxide                                            Extreme  zinc dialkyl dithiophosphate                                         Pressure tricresyl phosphate                                                  Additive sulfurized lard                                                      Solid    graphite (artificial                                                 Lubricant                                                                              mica (natural)                                                                boron nitride                                                                 MCA                                                                  Consistency Number (JIS K 2220)                                                                            --            --                                 E-12 Model Hot                                                                         400° C.                                                                          load resistance (kgf)                                                                   500    2000   500>                               Lubricating Per-   μ under above load                                                                   0.10   0.08   --                                 formance Test                                                                          600° C.                                                                          load resistance (kgf)                                                                   500    1000   500>                                                  μ under above load                                                                   0.16   0.10   --                                          800° C.                                                                          load resistance (kgf)                                                                   500>   500    500>                                                  μ under above load                                                                   --     0.10   --                                 Rust Prevention Test         ∘                                                                        ∘                                                                        ∘                      Water Washing Resistance Test (residual oil or                                                             31     55     --                                 grease ratio; %)                                                              Contact Heat Transfer Ratio between Metals                                                                 1      0.8-0.9                                                                              1                                  Roll Wearing Ratio in Actual Rolling Mill                                                                  1      0.5-0.6                                                                              1<                                 __________________________________________________________________________

Industrial Applicability

The lubricating composition of the present invention exerts an effect ofreducing the thermal crown of a work roll, which has an influence on thequality of a product, in the field of hot rolling steel, and isespecially valuable in this field.

We claim:
 1. A lubricating composition for hot-rolling steel, whichconsists essentially of a base oil or base grease; a combination ofheat-insulating agents (A) and (B), in which the heat-insulating agent(A) is an inorganic compound which is melted by an absorption of heat ata temperature lower than 1200° C., and the heat-insulating agent (B) isan inorganic powder which is not melted or decomposed at a temperaturelower than 1200° C. and has a heat conductivity lower than 0.01cal/cm.s. °C. at room temperature and a friction coefficient smallerthan 0.7; an extreme pressure additive; and a solid lubricant,whereinsaid combination of heat-insulating agents (A) and (B) is selected fromthe group consisting of a combination of a condensed phosphoric acidsalt as (A) and silicon nitride as (B); a combination of a condensedphosphoric acid salt as (A) and amorphous carbon as (B); a combinationof a condensed phosphoric acid salt as (A) and silicon dioxide as (B);and a combination of sodium chloride as (A) and silicon nitride as (B),said extreme pressure additive is selected from the group consisting ofzinc dialkyldithiophosphate, tricresyl phosphate and sulfurized lard,said solid lubricant is selected from the group consisting of graphite,molybdenum disulfide, mica and a melamine/cyanuric acid adduct, and saidheat-insulating agents (A) and (B) are contained in the composition in aweight ratio of from 49/1 to 1/49 and in a total amount of 5 to 50% byweight of the composition.
 2. A lubricating composition for hot-rollingsteel according to claim 1, wherein the base oil is a member selectedfrom the group consisting of spindle oil, machine oil, dynamo oil, motoroil, cylinder oil, bright stock, beef tallow, lard, sperm oil, palm oil,coconut oil, linseed oil, rice bran oil, soybean oil, esters of fattyacids having 8 to 22 carbon atoms with monohydric and polyhydricalcohols, α-olefins, polybutene, silicone oils and fluorine oils.
 3. Alubricating composition for hot-rolling steel according to claim 1,wherein the base grease is a member selected from the group consistingof lithium soap grease, calcium soap grease, sodium soap grease,aluminum soap grease, calcium complex grease, polyurea grease andorgano-clay grease.